Graphene oxide gel bonded graphene composite films and processes for producing same

ABSTRACT

Disclosed is a graphene composite thin film composition composed of nano graphene platelets (NGPs) bonded by a graphene oxide binder, wherein the NGPs contain single-layer graphene or multi-layer graphene sheets having a thickness from 0.335 nm to 100 nm. The NGPs occupy a weight fraction of 1% to 99.9% of the total composite weight. The graphene oxide binder, having an oxygen content of 1-40% (preferably &lt;10%) by weight based on the total graphene oxide weight, is obtained from a graphene oxide gel. The composite forms a thin film with a thickness no greater than 1 mm, but preferably no greater than 100 μm and no less than 10 μm. This composition has a combination of exceptional thermal conductivity, electrical conductivity, and mechanical strength unmatched by any thin-film material of comparable thickness range.

This invention is based on the results of a research project sponsoredby the US National Science Foundation SBIR-STTR Program.

FIELD OF THE INVENTION

The present invention relates generally to the field of graphiticmaterials for heat dissipation applications, and more particularly tographene composite films with an exceptionally high thermalconductivity, high electrical conductivity, and high mechanicalstrength.

BACKGROUND OF THE INVENTION

Carbon is known to have five unique crystalline structures, includingdiamond, fullerene (0-D nano graphitic material), carbon nano-tube (1-Dnano graphitic material), graphene (2-D nano graphitic material), andgraphite (3-D graphitic material).

The carbon nano-tube (CNT) refers to a tubular structure grown with asingle wall or multi-wall, which can be conceptually obtained by rollingup a graphene sheet or several graphene sheets to form a concentrichollow structure. Carbon nano-tubes have a diameter on the order of afew nanometers to a few hundred nanometers. Its longitudinal, hollowstructure imparts unique mechanical, electrical and chemical propertiesto the material.

A graphene sheet is composed of carbon atoms occupying a two-dimensionalhexagonal lattice. An isolated, individual graphene sheet is commonlyreferred to as single-layer graphene. A stack of multiple grapheneplanes bonded through van der Waals forces in the thickness direction iscommonly referred to as a multi-layer graphene, typically having 2-300layers or graphene planes, but more typically 2-100 graphene planes.Single-layer graphene and multi-layer graphene sheets are collectivelycalled “nano-scaled graphene platelets (NGPs).” Our research grouppioneered the development of graphene materials and related productionprocesses: (1) B. Z. Jang and W. C. Huang, “Nano-scaled GraphenePlates,” U.S. Pat. No. 7,071,258 (Jul. 4, 2006), application submittedin October 2012]; (2) B. Z. Jang, et al. “Process for ProducingNano-scaled Graphene Plates,” U.S. patent application Ser. No.10/858,814 (Jun. 3, 2004); and (3) Bor Z. Jang, Aruna Zhamu, andJiusheng Guo, “Process for Producing Nano-scaled Platelets andNanocomposites,” U.S. patent application Ser. No. 11/509,424 (Aug. 25,2006).

NGPs are typically obtained by intercalating natural graphite flakeswith a strong acid and/or oxidizing agent to obtain a graphiteintercalation compound (GIC), as illustrated in FIG. 1. This is mostoften accomplished by immersing natural graphite flakes in a mixture ofsulfuric acid, nitric acid (an oxidizing agent), and another oxidizingagent (e.g. potassium permanganate or sodium chlorate). The resultingGIC is actually some type of graphite oxide particles. This GIC is thenrepeatedly washed and rinsed in water to remove excess acids, resultingin a graphite oxide suspension or dispersion which contains discrete andvisually discernible graphite oxide particles dispersed in water. Thereare two processing routes to follow after this rinsing step:

Route 1 involves removing water from the suspension to obtain“expandable graphite,” which is essentially dried GIC or dried graphiteoxide particles. Upon exposure of expandable graphite to a temperaturein the range of 800-1050° C. for approximately 30 seconds to 2 minutes,the GIC expands by a factor of 30-300 to form a “graphite worm,” whichis a collection of exfoliated, but largely unseparated or interconnectedgraphite flakes. In Route 1A, these graphite worms (exfoliated graphiteor “networks of interconnected/non-separated graphite flakes”) can bere-compressed to obtain flexible graphite sheets that typically have athickness in the range of 0.125 mm (125 μm)-0.5 mm (500 μm). Theseflexible graphite (FG) sheets are used as a seal material and a heatspreader material, but exhibiting a maximum in-plane thermalconductivity of up to 600 W/mK (typically <300 W/mK) and in-planeelectrical conductivity no greater than 1,500 S/cm. In Route 1B, theexfoliated graphite is subjected to mechanical shearing (e.g. using anultrasonicator, high-shear mixer, air jet mill, or ball mill) to formseparated single-layer and multi-layer graphene sheets (collectively,NGPs), as disclosed in our U.S. application Ser. No. 10/858,814.

Route 2 entails ultrasonicating the graphite oxide suspension for thepurpose of separating individual graphene oxide sheets from graphiteoxide particles. This is based on the notion that the inter-grapheneplane separation bas been increased from 0.335 nm in natural graphite to0.6-1.1 nm in highly oxidized graphite oxide, significantly weakeningthe van der Waals forces that hold neighboring planes together.Ultrasonic power can be sufficient to further separate graphene planesheets to form separate, isolated, or discrete graphene oxide (GO)sheets. These graphene oxide sheets can then be chemically or thermallyreduced to obtain “reduced graphene oxides” (RGO) typically having anoxygen content of 2-10% by weight, more typically 2-5% by weight.

For the purpose of defining the claims of the instant application, NGPsinclude single-layer and multi-layer graphene or reduced graphene oxidewith an oxygen content of 0-10% by weight, more typically 0-5% byweight, and preferably 0-2% weight. Pristine graphene has essentially 0%oxygen.

It may be noted that flexible graphite sheets (obtained byre-compression of exfoliated graphite or graphite worms) exhibit arelatively low thermal conductivity (<600 W/mK as recited above).Flexible graphite sheets are also of low strength and poor structuralintegrity. The high tendency for flexible graphite sheets to get tornapart makes them difficult to handle in the process of integrating themin a microelectronic device.

Similarly, the NGPs, when packed into a film or paper sheet of non-wovenaggregates, exhibit a thermal conductivity higher than 1,000 W/mK onlywhen the film or paper is cast and pressed into a sheet having athickness lower than 10 μm, and higher than 1,500 W/mK only when thefilm or paper is cast and pressed into a sheet having a thickness lowerthan 1 μm. This is reported in our earlier U.S. patent application Ser.No. 11/784,606 (Apr. 9, 2007). However, ultra-thin film or paper sheets(<10 μm) are difficult to produce in mass quantities, and difficult tohandle when one tries to incorporate these thin films as a heat spreadermaterial during the manufacturing of microelectronic devices. Further,thickness dependence of thermal conductivity (not being able to achievea high thermal conductivity at a wide range of film thicknesses) is nota desirable feature.

Our earlier application (No. 11/784,606) further disclosed a mat, film,or paper of NGPs infiltrated with metal, glass, ceramic, resin, and CVDgraphite matrix material. Later on, Haddon, et al (US Pub. No.2010/0140792, Jun. 10, 2010) also reported NGP thin film and NGP-polymercomposites for thermal management applications. The processes used byHaddon et al to produce NGPs are identical to those disclosed muchearlier by us (Jang, et al. U.S. patent application Ser. No. 10/858,814(Jun. 3, 2004)). Balandin, et al (US Pub. No. 2010/0085713, Apr. 8,2010) also disclosed a graphene layer produced by CVD deposition ordiamond conversion for heat spreader application. More recently, Kim, etal (N. P. Kim and J. P. Huang, “Graphene Nanoplatelet Metal Matrix,” USPub. No. 2011/0108978, May 10, 2011) reported metal matrix infiltratedNGPs. However, metal matrix material is too heavy and the resultingmetal matrix composite does not exhibit a high thermal conductivity.

Another prior art material for thermal management application is thepyrolitic graphite film. The lower portion of FIG. 1 illustrates aprocess for producing prior art graphitic films or sheets. The processbegins with carbonizing a polymer 46 at a carbonization temperature of500-1,000° C. for 2-10 hours to obtain a carbonized material 48, whichis followed by a graphitization treatment at 2,500-3,200° C. for 5-24hours to form a graphitic film 50. This is a slow, tedious, andenergy-intensive process. Carbonization of certain polymers (e.g.polyacrylonitrile) involves the emission of toxic species.

Thus, it is an object of the present invention to provide a highlyconductive GO gel-bonded NGP composite thin-film structure (and relatedproduction processes) that exhibits a thermal conductivity greater than600 W/mK, typically greater than 800 W/mK, more typically greater than1,500 W/mK (even when the film thickness is greater than 10 μm), andmost preferably and often greater than 1,700 W/mK.

It is another object of the present invention to provide an NGP-GOcomposite thin-film sheet that exhibits a relativelythickness-independent thermal conductivity.

Still another object of the present invention is to provide a GO-bondedpristine graphene composite thin film that exhibits exceptional thermaland electrical conductivity properties.

It is a further object of the present invention to provide an NGP-GOcomposite thin-film sheet that is lightweight and exhibits a relativelyhigh strength or structural integrity.

It is yet another object of the present invention to provide a highlyconductive NGP-GO composite thin-film sheet (and related processes)wherein the in-plane thermal conductivity is greater than 600 W/mK(preferably and typically greater than 1,000 W/mK) and in planeelectrical conductivity is greater than 2,000 S/cm (preferably andtypically >3,000 S/cm), and/or a tensile strength greater than 10 MPa(preferably and typically >40 MPa).

It is another object of the present invention to provide a highlythermally conductive NGP-GO composite thin-film sheet that can be usedfor thermal management applications; e.g. for use as a heat spreader ina microelectronic device (such as mobile phone, notebook computer, andtablet), flexible display, light-emitting diode (LED), power tool,computer CPU, and power electronics. We are filing separate patentapplications to claim the various products or applications of thepresently invented NGP-GO composite thin-films.

SUMMARY OF THE INVENTION

The present invention provides a graphene composite thin filmcomposition composed of nano graphene platelets (NGPs) bonded by agraphene oxide binder. There is no other resin binder or matrix materialinvolved or included in this graphene composite. The NGPs containsingle-layer graphene or multi-layer graphene sheets having a thicknessfrom 0.335 nm to 100 nm, and the NGPs occupy a weight fraction of 1% to99.9% of the total composite weight. The graphene oxide binder, havingan oxygen content of 1-40% by weight based on the total graphene oxideweight, occupies a weight fraction of 0.1% to 99% of the total compositeweight. The composite forms a thin film with a thickness no greater than1 mm, preferably thinner than 200 μm, and further preferably not greaterthan 100 μm. The film thickness is preferably thicker than 10 μm, butcan be thinner. The multi-layer graphene sheets typically and preferablyhave a thickness of 3.35 nm to 33.5 nm and the film has a thicknesspreferably between 10 and 100 μm.

Preferably, the graphene oxide binder has an oxygen content of 1-10% byweight based on the total graphene oxide weight. The graphene oxidebinder preferably occupies a weight fraction of 1% to 40% of the totalcomposite weight. The graphene oxide may be obtained from a grapheneoxide gel. This gel is obtained by immersing a graphitic material in apowder or fibrous form in a strong oxidizing liquid in a reaction vesselat a reaction temperature for a length of time sufficient to obtain agraphene oxide gel.

This graphene oxide gel has the characteristics that it is opticallytransparent or translucent and visually homogeneous with no discerniblediscrete graphene or graphene oxide sheets dispersed therein. Incontrast, conventional suspension of discrete graphene or graphene oxidesheets, or graphite flakes looks opaque, dark, black or heavy brown incolor with individual graphene, graphene oxide sheets, or graphiteflakes being discernible or recognizable with naked eyes.

The graphene oxide molecules dissolved in a graphene oxide gel arearomatic chains that have an average number of benzene rings in thechain typically less than 1000, more typically less than 500, and mosttypically less than 100. Most of the molecules have more than 5 or 6benzene rings (mostly >10 benzene rings) from combined atomic forcemicroscopy, high-resolution TEM, and molecular weight measurements.These benzene-ring type of aromatic molecules have been heavily oxidizedand contain functional groups, such as —COOH and —OH and, therefore, are“soluble” (not just dispersible) in polar solvents, such as water.

These soluble molecules behave like polymers and are surprisinglycapable of serving as a binder or adhesive that bonds NGPs together toform a composite thin film of good structural integrity and high thermalconductivity. Conventional discrete graphene or graphene oxide sheets donot have any binding or adhesion power.

A preferred embodiment of the present invention is a graphene compositethin film composition that is obtained by mixing NGPs in a grapheneoxide gel to form a NGP-graphene oxide mixture suspension, making thesuspension into a thin film form, and removing the residual liquid fromthe mixture suspension. The resulting composite is composed of NGPs thatare bonded by a graphene oxide binder to form an essentially pore-freefilm or an integral film of low porosity level (having a physicaldensity greater than 1.4 g/cm³, more typically >1.6 g/cm³, andoften >1.8 g/cm³, approaching the theoretical density (2.25 g/cm³) ofgraphene sheets.

The graphene oxide binder is made from a graphene oxide gel obtained bydissolving a graphitic material in a fluid containing a strong oxidizingagent at a desired temperature for a length of time sufficient to form agel. The starting graphitic material for making graphene oxide gel maybe selected from natural graphite, artificial graphite, meso-phasecarbon, meso-phase pitch, meso-carbon micro-bead, soft carbon, hardcarbon, coke, carbon fiber, carbon nano-fiber, carbon nano-tube, or acombination thereof. The NGPs may also be produced from a graphiticmaterial selected from natural graphite, artificial graphite, meso-phasecarbon, meso-phase pitch, meso-carbon micro-beads, soft carbon, hardcarbon, coke, carbon fiber, carbon nano-fiber, carbon nano-tube, or acombination thereof.

The present invention further provides a bulk graphene compositecomposition composed of nano graphene platelets (NGPs) bonded by agraphene oxide binder obtained from a graphene oxide gel, wherein theNGPs contain single-layer graphene or multi-layer graphene sheets havinga thickness from 0.335 nm to 100 nm, and the NGPs occupy a weightfraction of 1% to 99.9% of the total composite weight; and the grapheneoxide binder, having an oxygen content of 0.1%-40% by weight based onthe total graphene oxide weight, occupies a weight fraction of 0.1% to99% of the total composite weight.

The present invention also provides a heat spreader or heat sink productfor use in a hand-held device, such as a power tool, a microelectronicor telecommunication device (e.g. mobile phone, tablet, laptop computer,LCD display, etc), a light-emitting diode (LED) lighting device orsystem. The light weight (lower density compared to metal and ceramicmaterials), exceptional thermal conductivity, and relatively highstructural integrity make the invented graphene oxide bonded NGPcomposite an ideal thermal management material.

The present invention also provides a process for producing a grapheneoxide-bonded graphene composite film. The process entails: (a) preparingsingle-layer or multilayer graphene platelets from a graphitic material;(b) preparing a graphene oxide gel having graphene oxide moleculesdispersed in a fluid medium, wherein the graphene oxide gel is opticallytransparent or translucent; (c) mixing the graphene platelets in thegraphene oxide gel to form a composite gel; and (d) forming thecomposite gel into the composite thin film by removing the fluid medium.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 A flow chart illustrating various prior art processes ofproducing exfoliated graphite products (flexible graphite sheets andflexible graphite composites) and pyrolytic graphite films, along withpresently invented processes of producing a graphene oxide gel-bondedNGP composite.

FIG. 2 (a) A SEM image of a graphite worm sample after exfoliation ofgraphite intercalation compounds (GICs); (b) An SEM image of across-section of a flexible graphite sheet, showing many graphite flakeswith orientations not parallel to the flexible graphite sheet surfaceand many defect structures.

FIG. 3 (A) Thermal conductivity, (B) electrical conductivity, and (C)tensile strength of GO gel bonded NGP composites as a function of the GOcontent in the composite.

FIG. 4 Thermal conductivity values of GO gel bonded NGP composites andpolyimide-based graphitic films as a function of the heat treatmenttemperature.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Graphite is made up of layer planes of hexagonal networks of carbonatoms. These layer planes of hexagonally arranged carbon atoms aresubstantially flat and are oriented or ordered so as to be substantiallyparallel and equidistant to one another. These layers of carbon atoms,commonly referred to as graphene layers or basal planes, are wealdybonded together in their thickness direction by weak van der Waalsforces and groups of these graphene layers are arranged in crystallites.Highly ordered graphites consist of crystallites of considerable size:the crystallites being highly aligned or oriented with respect to eachother and having well ordered carbon layers. These anisotropicstructures give rise to many properties that are highly directional,such as thermal and electrical conductivity.

The graphite structure is usually characterized in terms of two axes ordirections: the “c” axis or direction and the “a” axes or directions.The “c” axis is the direction perpendicular to the basal planes. The “a”axes are the directions parallel to the basal planes (perpendicular tothe “c” direction). The graphite suitable for manufacturing flexiblegraphite sheets is typically natural graphite flakes that possess a highdegree of orientation.

Due to the weak van der Waals forces holding the parallel graphenelayers, natural graphite can be treated so that the spacing between thegraphene layers can be appreciably opened up so as to provide a markedexpansion in the “c” direction, and thus form an expanded graphitestructure in which the laminar character of the carbon layers issubstantially retained. The process for manufacturing flexible graphiteis well-known and the typical practice is described in U.S. Pat. No.3,404,061 to Shane et al., the disclosure of which is incorporatedherein by reference. In general, flakes of natural graphite areintercalated in an acid solution to produce graphite intercalationcompounds (GICs). The GICs are washed, dried, and then exfoliated byexposure to a high temperature for a short period of time. This causesthe flakes to expand or exfoliate in the “c” direction of the graphiteup to 80-300 times of their original dimensions. The exfoliated graphiteflakes are vermiform in appearance and, hence, are commonly referred toas worms. These worms of graphite flakes which have been greatlyexpanded can be formed without the use of a binder into cohesive orintegrated sheets of expanded graphite, e.g. webs, papers, strips,tapes, foils, mats or the like (typically referred to as “flexiblegraphite”) having a typical density of about 0.04-2.0 g/cm³ for mostapplications.

The upper left portion of FIG. 1 shows a flow chart that illustrates theprior art processes used to fabricate flexible graphite and theresin-impregnated flexible graphite composite. The processes typicallybegin with intercalating graphite particles 20 (e.g., natural graphiteor synthetic graphite flakes) with an intercalant (typically a strongacid or acid mixture) to obtain a graphite intercalation compound 22(GIC). After rinsing in water to remove excess acid, the GIC becomes“expandable graphite.” The GIC or expandable graphite is then exposed toa high temperature environment (e.g., in a tube furnace preset at atemperature in the range of 800-1,050° C.) for a short duration of time(typically from 15 seconds to 2 minutes). This thermal treatment allowsthe graphite to expand in its “c” direction by a factor of 30 to severalhundreds to obtain a worm-like vermicular structure, which containsexfoliated, but unseparated graphite flakes 24 with large poresinterposed between these interconnected flakes.

In one prior art process, the exfoliated graphite is re-compressed byusing a calendering or roll-pressing technique to obtain flexiblegraphite sheets or foils 26, which are typically much thicker than 100μm. It seems that no flexible graphite sheet thinner than 75 μm has everbeen reported in the open literature or patent documents. Commerciallyavailable flexible graphite sheets normally have a thickness greaterthan 0.125 mm (125 μm), an in-plane electrical conductivity of 1−3×10³S/cm, through-plane (thickness-direction) electrical conductivity of15-30 S/cm, in-plane thermal conductivity of 140-190 W/(mK), andthrough-plane thermal conductivity of approximately 5 W/(mK). Theseproperties are inadequate for many thermal management applications andthe present invention is made to address these issues.

In another prior art process, the exfoliated graphite worm 24 may beimpregnated with a resin and then compressed and cured to form aflexible graphite composite 28, which is normally of low strength.

The exfoliated graphite may be subjected to high-intensity mechanicalshearing/separation treatments using an air mill, ball mill, orultrasonic device to produce separated nano-scaled graphene plates 34(NGPs) with all the graphite platelets thinner than 100 nm, mostlythinner than 10 nm. An NGP is composed of a graphene sheet or aplurality of graphene sheets with each sheet being a two-dimensional,hexagonal carbon structure.

For the purpose of defining the geometry and orientation of an NGP, theNGP is described as having a length (the largest dimension), a width(the second largest dimension), and a thickness. The thickness is thesmallest dimension, which is no greater than 100 nm, preferably smallerthan 10 nm in the present application. When the platelet isapproximately circular in shape, the length and width are referred to asdiameter. In the presently defined NGPs, both the length and width canbe smaller than 1 μm, but can be larger than 200 μm.

In addition to graphene or NGPs, another ingredient of the presentlyinvented composite thin film composition is graphene oxide (GO), whichis obtained from a graphene oxide gel. This gel is obtained by immersinga graphitic material 20 in a powder or fibrous form in a strongoxidizing liquid in a reaction vessel to form an optically opaquesuspension or slurry. The reaction between graphite powder and theoxidizing agent is allowed to proceed at a reaction temperature for alength of time sufficient to transform this opaque suspension into atranslucent or transparent solution, which is now a homogeneous fluidcalled “graphene oxide gel.”

This graphene oxide gel is optically transparent or translucent andvisually homogeneous with no discernible discrete graphene or grapheneoxide sheets dispersed therein. In contrast, conventional suspension ofdiscrete graphene sheets, graphene oxide sheets, and expanded graphiteflakes look dark, black or heavy brown in color with individual grapheneor graphene oxide sheets or expanded graphite flakes discernible orrecognizable with naked eyes.

The graphene oxide molecules dissolved in a graphene oxide gel arearomatic chains that have an average number of benzene rings in thechain typically less than 1000, more typically less than 500, and mosttypically less than 100. Most of the molecules have more than 5 or 6benzene rings (mostly >10 benzene rings) from combined atomic forcemicroscopy, high-resolution TEM, and molecular weight measurements.Based on our elemental analysis, these benzene-ring type of aromaticmolecules are heavily oxidized, containing a high concentration offunctional groups, such as —COOH and —OH and, therefore, are “soluble”(not just dispersible) in polar solvents, such as water. The estimatedmolecular weight of these graphene oxide polymers in the gel state istypically between 200 g/mole and 43,000 g/mole, more typically between400 g/mole and 21,500 g/mole, and most typically between 400 g/mole and4,000 g/mole.

These soluble molecules behave like polymers and are surprisinglycapable of serving as a binder or adhesive that bonds NGPs together toform a composite thin film of good structural integrity and high thermalconductivity. Conventional discrete graphene or graphene oxide sheets donot have any binding or adhesion power.

The present invention provides a graphene composite thin filmcomposition composed of nano graphene platelets (NGPs) bonded by agraphene oxide binder, wherein the NGPs contain single-layer graphene ormulti-layer graphene sheets having a thickness from 0.335 nm to 100 nm,and the NGPs occupy a weight fraction of 1% to 99.9% of the totalcomposite weight; and the graphene oxide binder (having an oxygencontent of 1-40% by weight based on the total graphene oxide weight)occupies a weight fraction of 0.1% to 99% of the total composite weight,and wherein the composite forms a thin film with a thickness no greaterthan 1 mm, preferably less than 200 μm, further preferably less than 100μm. More preferably, the thickness is greater than 10 μm, furtherpreferably between 10 and 100 μm, and most preferably between 10 μm and50 μm. A thickness less than 10 μm would make it impossible to handlethe composite thin film when attempting to incorporate pieces of thecomposite thin film in a device for thermal management applications(e.g. as a heat spreader in a microelectronic device).

The multi-layer graphene sheets preferably have a thickness of 3.35 nmto 33.5 nm and the resulting composite film has a thickness no greaterthan 100 μm. When multi-layer graphene sheets have a thickness of 6.7 nmto 20 nm, one can readily produce a composite film having a thicknessnot greater than 50 μM.

The graphene oxide-bonded graphene composite thin film compositiondesirably contains pristine graphene containing no oxygen. The pristinegraphene can be obtained from direct ultrasonication without involvingoxidation of a graphitic material.

The graphene oxide (GO) binder has an oxygen content of 1-10% by weightbased on the total graphene oxide weight. The GO binder, when in a gelstate, typically has an oxygen content of 20-46% by weight. Aftercombining with NGPs to form a composite thin film, the process naturallyreduces the oxygen content to typically 1-10% by weight.

The graphene oxide binder occupies a weight fraction of 1% to 20% of thetotal composite weight. The graphene oxide is obtained from a grapheneoxide gel, which gel is composed of graphene oxide molecules dispersedin an acidic medium having a pH value of no higher than 5 and thegraphene oxide molecules have an oxygen content no less than 20% byweight. In particular, the gel is obtained by immersing a graphiticmaterial in a powder or fibrous form in an oxidizing liquid in areaction vessel at a reaction temperature for a length of timesufficient to obtain a graphene oxide gel composed of graphene oxidemolecules dispersed in an acidic medium having a pH value of no higherthan 5 and the graphene oxide molecules have an oxygen content no lessthan 20% by weight. The graphene composite is obtained by mixing theNGPs in the graphene oxide gel to form a NGP-graphene oxide mixturesuspension, making the suspension into a thin film form, and removingthe residual liquid from the mixture suspension.

The starting graphitic material for the purpose of forming grapheneoxide gel may be selected from natural graphite, artificial graphite,meso-phase carbon, meso-phase pitch, meso-carbon micro-bead, softcarbon, hard carbon, coke, carbon fiber, carbon nano-fiber, carbonnano-tube, or a combination thereof. The graphitic material ispreferably in a powder or short filament form having a dimension lowerthan 20 μm, more preferably lower than 10 μm, further preferably smallerthan 5 μm, and most preferably smaller than 1 μm.

Using artificial graphite with an average particle size of 9.7 μm as anexample, a typical procedure involves dispersing graphite particles inan oxidizer mixture of sulfuric acid, nitric acid, and potassiumpermanganate (at a weight ratio of 3:1:0.05) at a temperature oftypically 0-60° C. for typically at least 3 days, preferably 5 days, andmore preferably 7 days or longer. The average molecular weight of theresulting graphene oxide molecules in a gel is approximately20,000-40,000 g/mole if the treatment time is 3 days, <10,000 g/mole if5 days, and <4,000 g/mole if longer than 7 days. The required gelformation time is dependent upon the particle size of the originalgraphitic material, a smaller size requiring a shorter time. It is offundamental significance to note that if the critical gel formation timeis not reached, the suspension of graphite powder and oxidizer (graphiteparticles dispersed in the oxidizer liquid) appears completely opaque,meaning that discrete graphite particles remain suspended (but notdissolved) in the liquid medium. As soon as this critical time isexceeded, the whole suspension becomes optically translucent ortransparent, meaning that the heavily oxidized graphite completely lostits original graphite identity and the resulting graphene oxidemolecules are completely dissolved in the oxidizer liquid, forming ahomogeneous solution (no longer just a suspension or slurry).

It must be further noted that if the suspension or slurry, with atreatment time being shorter than the required gel formation time, isrinsed and dried, we would simply recover a graphite oxide powder orgraphite intercalation compound (GIC) powder, which can be exfoliatedand separated to produce nano graphene platelets (NGPs).

Hence, the NGPs may be produced by subjecting a graphitic material to acombined treatment of oxidation, exfoliation, and separation. Thisgraphitic material may also be selected from natural graphite,artificial graphite, meso-phase carbon, meso-phase pitch, meso-carbonmicro-bead, soft carbon, hard carbon, coke, carbon fiber, carbonnano-fiber, carbon nano-tube, or a combination thereof. The NGPs canalso be produced from a process such as (a) direct ultrasonication, (b)potassium melt intercalation and water/alcohol-induced exfoliation, or(c) supercritical fluid intercalation/exfoliation/separation ofnon-oxidized graphitic material. These processes produce pristinegraphene that contains no oxygen

The graphene composite thin film composition typically has a thermalconductivity greater than 800 Wm⁻¹K⁻¹, more typically greater than 1,000Wm⁻¹K⁻¹ (even when the film thickness is greater than 10 μm) and oftengreater than 1,700 Wm⁻¹K⁻¹. The composite thin film has an electricalconductivity greater than 3,000 S/cm. This high electrical conductivity(greater than 3000 S/cm) can be achieved concurrently with a thermalconductivity greater than 1,000 Wm⁻¹K⁻¹. Quite often, the composite thinfilm can exhibit a combination of a high electrical conductivity(greater than 1,500 S/cm), a high thermal conductivity (greater than 600Wm⁻¹K⁻¹), a relatively high physical density (greater than 1.4 g/cm³),and a relatively high tensile strength (greater than 10 MPa).

Quite surprisingly, in many samples, the composite thin film has anelectrical conductivity greater than 2,000 S/cm, a thermal conductivitygreater than 800 Wm⁻¹K⁻¹, a physical density greater than 1.8 g/cm³, anda tensile strength greater than 40 MPa. This combination of superiorproperties has not been achieved with any graphite or non-graphitematerial. In some cases, the composite thin film has an electricalconductivity greater than 3,000 S/cm, a thermal conductivity greaterthan 1,500 Wm⁻¹K⁻¹, a physical density greater than 2.0 g/cm³, and atensile strength greater than 40 MPa. This type of graphene compositefilm may be used as a heat spreader component in a portable device.

The present invention also provides a process for producing a grapheneoxide-bonded graphene composite film. The process comprises (a)preparing single-layer or multilayer graphene platelets from a graphiticmaterial; (b) preparing a graphene oxide gel having graphene oxidemolecules dispersed in an acidic fluid medium; (c) mixing the grapheneplatelets in the graphene oxide gel to form a composite gel; and (d)forming a composite gel into a composite thin film by removing the fluidmedium.

The graphene platelets preferably are pristine graphene containing nooxygen. The pristine graphene is prepared from a graphitic materialwithout involving oxidation of graphite.

As illustrated in FIG. 1, the graphene oxide gel is prepared byimmersing a graphitic material in a powder or fibrous form in a strongoxidizing liquid in a reaction vessel at a reaction temperature for alength of time sufficient to obtain a graphene oxide gel composed ofgraphene oxide molecules dispersed in an acidic medium having a pH valueof no higher than 5 and the graphene oxide molecules have an oxygencontent no less than 20% by weight. Such a graphitic material isselected from natural graphite, artificial graphite, meso-phase carbon,meso-phase pitch, meso-carbon micro-bead, soft carbon, hard carbon,coke, carbon fiber, carbon nano-fiber, carbon nano-tube, or acombination thereof.

The graphene platelets may be produced from a graphitic materialselected from natural graphite, artificial graphite, meso-phase carbon,meso-phase pitch, meso-carbon micro-bead, soft carbon, hard carbon,coke, carbon fiber, carbon nano-fiber, carbon nano-tube, or acombination thereof. The starting material may be immersed in a mixtureof sulfuric acid, nitric acid, and potassium permanganate to allow forintercalation of acid into interior of the graphitic, resulting in theformation of graphite intercalation compound (GIC) after rinsing anddrying. The GIC is then exfoliated in a high temperature furnace toobtain exfoliated graphite, which is then subjected to mechanicalshearing to obtain isolated NGPs.

For the preparation of graphene oxide gel-bonded graphene composite thinfilm, the discrete NGPs (preferably thinner than 20 nm, more preferablythinner than 10 nm) are dispersed in a graphene oxide gel to produce asuspension wherein discrete graphene platelets (NGPs) are suspended inthe oxidizer mixture liquid 36 (FIG. 1), in which graphene oxidemolecules are dissolved as well. The graphene platelet concentration ispreferably lower than 50% by weight in the suspension (most preferablysmaller than 20%). The suspension (or slurry) is allowed to form intothin film structures using techniques like solvent cast, vacuum-assistedfiltration, spin coating, dip coating, or paper-making. Upon removal ofthe liquid medium, the resulting thin-film structures 40 containdiscrete graphene platelets being dispersed in a polymer binder matrix(graphene oxide molecules). This thin-film structure is then subjectedto a thermal treatment or re-graphitization treatment (typically100-1000° C., but can be higher), which allows individual graphene oxidemolecules to chemically bond to graphene sheets or platelets. Thisthermal treatment also surprisingly enables or activates the re-joining,polymerization, or chain-growth of otherwise small graphene oxidemolecules, resulting in removal of non-carbon elements (e.g. H and O)and formation of large graphene sheets. The presence of discrete NGPsprovides “nucleation sites” to accelerate the growth of these hugegraphene sheets. It appears that the original NGPs (discrete grapheneplatelets) and graphene oxide molecules can be merged and integratedinto one unitary entity that exhibits an unprecedented combination ofexceptional thermal conductivity, electrical conductivity, structuralintegrity (strength and ease of handling). These properties areunmatched by any graphitic or non-graphitic materials.

The thermal treatment process can be assisted with a calendering orroll-pressing operation to help improve the surface finish of theresulting thin film. The film thickness can be less than 10 μm, butpreferably between 10 μm and 100 μm.

Thus, the present invention also provides a process for producinggraphene-oxide-bonded pristine graphene composite 42 (FIG. 1), whichinvolves mixing discrete NGPs in said graphene oxide gel to form aNGP-graphene oxide mixture suspension 38, making the suspension into athin film form, removing a residual liquid from the mixture suspension,and subjecting the thin film 42 to a re-graphitization treatment at atemperature in the range of 100° C. and 3,200° C. The re-graphitizationtemperature is preferably in the range of 300° C. and 1,500° C. or inthe range of 300° C. and 1,000° C. The thin film composition can forminto a unitary structure after the re-graphitization treatment.

As indicated above, flexible graphite sheets prepared by re-compressionof exfoliated graphite or graphite worms exhibit relatively low thermalconductivity and mechanical strength. The graphite worms can be formedinto flexible graphite sheets by compression, without the use of anybinding material, presumably due to the mechanical interlocking betweenthe voluminously expanded graphite flakes. Although a significantproportion of these flakes are oriented in a direction largely parallelto the opposing surfaces of a flexible graphite sheet (as evidenced bythe high degree of anisotropy with respect to thermal and electricalconductivity), many other flakes are distorted, kinked, bent over, ororiented in a direction non-parallel to these sheet surfaces. Thisobservation has been well demonstrated in many scanning electronmicrographs (SEM) published in open or patent literature. Furthermore,the presence of a large number of graphite flakes implies a large amountof interface between flakes, resulting in very high contact resistance(both thermal and electrical resistance).

As a consequence, the electrical or thermal conductivity of theresulting flexible graphite sheets dramatically deviates from what wouldbe expected of a perfect graphite single crystal or a graphene layer.For instance, the theoretical in-plane electrical conductivity andthermal conductivity of a graphene layer are predicted to be 1-5×10⁴S/cm and 3,000-5,000 W/(mK), respectively. However, the actualcorresponding values for flexible graphite are 1-3×10³ S/cm and 140-300W/(mK), respectively; one order of magnitude lower than what could beachieved. By contrast, the corresponding values for the presentlyinvented graphene-oxide bonded graphene composite films are 3.5-10×10³S/cm and 600-2,230 W/(mK), respectively.

The present invention also provides a highly thermally conductive NGP-GOcomposite thin-film sheet that can be used for thermal managementapplications; e.g. for use as a heat spreader in a microelectronicdevice (such as mobile phone, notebook computer, e-book, and tablet),flexible display, light-emitting diode (LED), power tool, computer CPU,and power electronics. We are filing separate patent applications toclaim the various products or applications of the presently inventedNGP-GO composite thin-films.

Example 1 Preparation of Nano Graphene Platelets (NGPs)

Chopped graphite fibers with an average diameter of 12 μm was used as astarting material, which was immersed in a mixture of concentratedsulfuric acid, nitric acid, and potassium permanganate (as the chemicalintercalate and oxidizer) to prepare graphite intercalation compounds(GICs). The fiber segments were first dried in a vacuum oven for 24 h at80° C. Then, a mixture of concentrated sulfuric acid, fuming nitricacid, and potassium permanganate (at a weight ratio of 4:1:0.05) wasslowly added, under appropriate cooling and stirring, to a three-neckflask containing fiber segments. After 16 hours of reaction, theacid-treated graphite fibers were filtered and washed thoroughly withdeionized water until the pH level of the solution reached 6. Afterbeing dried at 100° C. overnight, the resulting graphite intercalationcompound (GIC) was subjected to a thermal shock at 1050° C. for 45seconds in a tube furnace to form exfoliated graphite (worms). Fivegrams of the resulting exfoliated graphite (EG) were mixed with 2,000 mlalcohol solution consisting of alcohol and distilled water with a ratioof 65:35 for 12 hours to obtain a suspension. Then the mixture orsuspension was subjected to ultrasonic irradiation with a power of 200 Wfor various times. After two hours of sonication, EG particles wereeffectively fragmented into thin NGPs. The suspension was then filteredand dried at 80° C. to remove residue solvents. The as-prepared NGPshave an average thickness of approximately 9.7 nm.

Example 2 Preparation of Single-Layer Graphene from Meso-CarbonMicro-Beads (MCMBs)

Meso-carbon microbeads (MCMBs) were supplied from China Steel ChemicalCo. This material has a density of about 2.24 g/cm³ with a medianparticle size of about 16 μm. MCMB (10 grams) were intercalated with anacid solution (sulfuric acid, nitric acid, and potassium permanganate ata ratio of 4:1:0.05) for 72 hours. Upon completion of the reaction, themixture was poured into deionized water and filtered. The intercalatedMCMBs were repeatedly washed in a 5% solution of HCl to remove most ofthe sulphate ions. The sample was then washed repeatedly with deionizedwater until the pH of the filtrate was neutral. The slurry was dried andstored in a vacuum oven at 60° C. for 24 hours. The dried powder samplewas placed in a quartz tube and inserted into a horizontal tube furnacepre-set at a desired temperature, 1,080° C. for 45 seconds to obtain agraphene material. TEM and atomic force microscopic studies indicatethat most of the NGPs were single-layer graphene.

Example 3 Preparation of Pristine Graphene

In a typical procedure, five grams of graphite flakes, ground toapproximately 20 μm or less in sizes, were dispersed in 1,000 mL ofdeionized water (containing 0.1% by weight of a dispersing agent, Zonyl®FSO from DuPont) to obtain a suspension. An ultrasonic energy level of85 W (Branson 5450 Ultrasonicator) was used for exfoliation, separation,and size reduction of graphene sheets for a period of 15 minutes to 2hours.

Example 4 Preparation of Graphene Oxide Gel

Graphite oxide gel was prepared by oxidation of graphite flakes with anoxidizer liquid consisting of sulfuric acid, sodium nitrate, andpotassium permanganate at a ratio of 4:1:0.05 at 30° C. When naturalgraphite flakes (particle sizes of 14 μm) were immersed and dispersed inthe oxidizer mixture liquid, the suspension or slurry appears opticallyopaque and dark. The suspension remains opaque during the first 52 hoursof reaction. However, the suspension gradually turns opticallytranslucent (a little cloudy) when the reaction time exceeds 52 hours,and the color of the suspension changes from black to dark brown. After96 hours, the suspension suddenly becomes an optically transparentsolution with light brown color. The solution appears very uniform incolor and transparency, indicating the absence of any dispersed discreteobjects. The whole solution behaves like a gel, very similar to atypical polymer gel.

Surprisingly, by casting this gel on a glass surface and removing theliquid medium from the cast film we obtain a thin film of graphene oxidethat is optically transparent. This thin film looks like and behaveslike a regular polymer film.

Example 5 Preparation of Graphene Oxide Bonded Graphene CompositeThin-Film

The NGPs prepared in Examples 1-3 and the graphene oxide gel prepared inExample 4 were used for the preparation of graphene oxide-bondedgraphene composite. Fully separated NGP platelets were dispersed ingraphene oxide gel to produce a graphene platelet suspension with theplatelet concentration of approximately 1-50% by weight (preferably5-20% by weight NGP). Ultrasonic waves were employed to assist in thedispersion of NGPs in the gel. This NGP-gel suspension or slurry wasthen cast onto a glass surface and regulated by a doctor's blade to forma film of uniform thickness. The liquid in the film was then removed ina vacuum oven to form a solid composite film.

Some selected solid films were subjected to a heat treatment(re-graphitization treatment) at a temperature of 100-1,000° C. (in somecases, 1,500-2,800° C., for comparison purposes). For comparison, wealso carbonized polyimide films at 1000° C. for 3 hours in an inertatmosphere and then graphitized the films at a temperature in the rangeof 2,500-3,000° C. for 5 hours to form a conventional graphitic film.Flexible graphite sheets were also obtained from commercial sources asanother baseline material.

The in-plane thermal and electrical conductivities and tensileproperties of various films were investigated. Several significantobservations can be made from the testing results (e.g. as summarized inFIG. 3(A), (B), (C) and FIG. 4):

-   -   (1) At a thickness of approximately 45 μm the thermal        conductivity of the graphene oxide-bonded NGP composite films        (heat treated at 800° C.) increases from 720 W/(mK) at 0% GO        binder (containing graphene sheets only) to reach a maximum of        1310 W/(mK) at 30% GO, as shown in FIG. 3(A). The thermal        conductivity value begins to decrease with a further increase in        the GO binder amount. This maximum thermal conductivity is        significantly higher than that (720 W/mK) of the thin film made        up of NGPs only (0% GO) and that of the film containing GO only        (100% GO). These data have clearly demonstrated an un-expected,        synergistic effect between NGP (graphene) and GO (graphene        oxide).    -   (2) These graphene-based composite thin films exhibit much        higher thermal conductivity values than those (typically 140-300        W/(mK)) of commercially available flexible graphite sheets.    -   (3) FIG. 3(B) also shows a dramatic synergistic effect in        electrical conductivity when graphene is combined with graphene        oxide to form a composite material.    -   (4) Another unexpected observation is the notion that the        tensile strength of the GO-NGP composite increases monotonically        with the increasing GO content. This appears to suggest that GO        gel has a strong adhering power capable of bonding graphene        sheets or integrating with graphene sheets together, and that GO        molecules in a GO gel are capable of combining with one another        to form larger and stronger graphene/graphene oxide sheets that        are relatively defect-free, leading to a relatively high        mechanical strength.    -   (5) The presently invented GO-NGP composite can be readily        produced into thin films much thinner than 100 μm (the practical        lower limit of flexible graphite sheet thickness). Within the        thickness range of 10-100 μm, the GO-NGP composite films exhibit        an exceptionally high thermal conductivity of >1800 W/(mK) and        the thermal conductivity is relatively independent of the film        thickness (1820 W/mK at 24 μm, 1831 W/mK at 54 μm, and 1826 W/mK        at 72 μm in thickness). It may be noted that NGPs alone (without        GO gel as a binder), when packed into a film or paper sheet of        non-woven aggregates, exhibit a thermal conductivity higher than        1,000 W/mK only when the film or paper is cast and pressed into        a sheet having a thickness lower than 10 μm, and higher than        1,500 W/mK only when the film or paper is cast and pressed into        a sheet having a thickness lower than 1 μm. This was reported in        our earlier U.S. patent application Ser. No. 11/784,606 (Apr. 9,        2007). However, ultra-thin film or paper sheets (<10 μm) are        difficult to produce in mass quantities, and difficult to handle        when one tries to incorporate these thin films as a heat        spreader material during the manufacturing of microelectronic        devices. Further, thickness dependence of thermal conductivity        (not being able to achieve a high thermal conductivity at a wide        range of film thicknesses) is not a desirable feature.    -   (6) FIG. 3(A) also demonstrates that GO-bonded pristine graphene        composite exhibits significantly higher thermal conductivity as        compared to GO-bonded non-pristine NGP composites. The NGPs        prepared from chemical oxidation or intercalation processes tend        to have structural defects on graphene plane and have some        non-carbon elements (e.g. oxygen and hydrogen). These NGPs are        distinct from pristine NGPs (e.g. prepared from direct        ultrasonication of graphite, alkali metal intercalation, and        supercritical fluid extraction) in terms of chemical        composition, microstructure, and properties).    -   (7) As indicated in FIG. 4, the presently invented NGP-GO        composite compositions do not have to go through an        ultra-high-temperature graphitization treatment. Graphitization        of a carbonized resin (e.g. polyimide) or other carbon materials        requires a temperature typically higher than 2,000° C., most        typically higher than 2,500° C. The graphitization temperature        is most typically in the range of 2,800-3,200° C. in order for        carbonized materials or pyrolytic graphite to achieve a thermal        conductivity >1,600 W/mK. In contrast, the typical heat        treatment temperature (re-graphitization treatment) of the        presently invented NGP-GO composites is significantly lower than        1,500° C. and more typically lower than 1,000° (can be as low as        100° C.). For instance, carbonized polyimide, if graphitized at        2,000° C. for 5 hours, exhibits a thermal conductivity of 820        W/mK. In contrast, we were able to reach a thermal conductivity        of 876 W/mK with a heat treatment of NGP-GO at 500° C. for two        hours. This is very surprising and no one has ever thought that        such a low graphitization temperature was possible. Clearly,        this is a dramatically faster, less energy-intensive, and more        cost-effective process.

In conclusion, we have successfully developed a new and novel class ofhighly conducting graphene oxide gel-bonded composites that containnon-pristine or pristine graphene sheets. The thermal and electricalconductivities and tensile strength exhibited by the presently inventedmaterials are much higher than what prior art flexible graphite sheetsor other graphitic films could achieve. The thermal and electricalconductivities exhibited by the presently invented materials are thehighest of what graphite-type thin-layer materials (>10 μm) couldachieve.

We claim:
 1. A graphene composite thin film composition composed of nanographene platelets (NGPs) bonded by a graphene oxide binder, whereinsaid NGPs contain single-layer graphene or multi-layer graphene sheetshaving a thickness from 0.335 nm to 100 nm, and said NGPs occupy aweight fraction of 1% to 99.9% of the total composite weight; and saidgraphene oxide binder, having an oxygen content of 0.1%-40% by weightbased on the total graphene oxide weight, occupies a weight fraction of0.1% to 99% of the total composite weight, and wherein said compositeforms a thin film with a thickness no greater than 1 mm.
 2. The graphenecomposite thin film composition as defined in claim 1, which has athickness less than 100 μm.
 3. The graphene composite thin filmcomposition as defined in claim 1, which has a thickness greater than 10μm, but less than 100 μm.
 4. The graphene composite thin filmcomposition as defined in claim 1 wherein said multi-layer graphenesheets have a thickness of 6.7 nm to 20 nm or said film has a thicknessnot greater than 50 μm.
 5. The graphene composite thin film compositionas defined in claim 1, wherein said nano graphene platelets are pristinegraphene containing no oxygen.
 6. The graphene composite thin filmcomposition as defined in claim 1, wherein said nano graphene plateletsare pristine graphene containing no oxygen and said pristine graphene isobtained from an oxidation-free procedure selected from directultrasonication, supercritical fluid intercalation, or alkali metalintercalation.
 7. The graphene composite thin film composition asdefined in claim 1 wherein said graphene oxide binder has an oxygencontent of 1-10% by weight based on the total graphene oxide weightwhich is measured after said graphene composite film composition ismade.
 8. The graphene composite thin film composition as defined inclaim 1 wherein said graphene oxide binder occupies a weight fraction of1% to 40% of the total composite weight.
 9. The graphene composite thinfilm composition as defined in claim 1 wherein said graphene oxide isobtained from a graphene oxide gel, which gel is composed of grapheneoxide molecules dispersed in an acidic medium having a pH value of nohigher than 5 and said graphene oxide molecules have an oxygen contentno less than 20% by weight while in a gel state.
 10. The graphenecomposite thin film composition as defined in claim 1 wherein saidgraphene oxide is obtained from a graphene oxide gel, which gel isobtained by immersing a graphitic material in a powder or fibrous formin an oxidizing liquid medium in a reaction vessel at a reactiontemperature for a length of time sufficient to obtain a graphene oxidegel composed of graphene oxide molecules dispersed in the liquid mediumand said graphene oxide molecules have an oxygen content no less than20% by weight and a molecular weight less than 43,000 g/mole while in agel state.
 11. The graphene composite thin film composition as definedin claim 10 wherein said graphene oxide molecules have a molecularweight less than 4,000 g/mole.
 12. The graphene composite thin filmcomposition as defined in claim 10 wherein said graphene oxide moleculeshave a molecular weight between 200 g/mole and 4,000 g/mole.
 13. Thegraphene composite thin film composition as defined in claim 10 whereinsaid graphene composite is obtained by mixing said NGPs in said grapheneoxide gel to form a NGP-graphene oxide mixture suspension, making saidsuspension into a thin film form, and removing a residual liquid fromsaid mixture suspension.
 14. The graphene composite thin filmcomposition as defined in claim 10 wherein said graphene composite isobtained by mixing said NGPs in said graphene oxide gel to form aNGP-graphene oxide mixture suspension, making said suspension into athin film form, removing a residual liquid from said mixture suspension,and subjecting to a re-graphitization treatment at a temperature in therange of 100° C. and 3,200° C.
 15. The graphene composite thin filmcomposition as defined in claim 14 wherein said re-graphitizationtemperature is in the range of 300° C. and 1,500° C.
 16. The graphenecomposite thin film composition as defined in claim 14 wherein saidre-graphitization temperature is in the range of 100° C. and 1,000° C.17. The graphene composite thin film composition as defined in claim 14wherein said thin film composition forms into a unitary structure aftersaid re-graphitization treatment.
 18. The graphene composite thin filmcomposition as defined in claim 10 wherein said graphitic material isselected from natural graphite, artificial graphite, meso-phase carbon,meso-phase pitch, meso-carbon micro-bead, soft carbon, hard carbon,coke, carbon fiber, carbon nano-fiber, carbon nano-tube, or acombination thereof.
 19. The graphene composite thin film composition asdefined in claim 1 wherein said NGPs are produced from a graphiticmaterial selected from natural graphite, artificial graphite, meso-phasecarbon, meso-phase pitch, meso-carbon micro-bead, soft carbon, hardcarbon, coke, carbon fiber, carbon nano-fiber, carbon nano-tube, or acombination thereof.
 20. The graphene composite thin film composition asdefined in claim 1 wherein said thin film has a thermal conductivitygreater than 1,000 Wm⁻¹K⁻¹.
 21. The graphene composite thin filmcomposition as defined in claim 1 wherein said thin film has a thermalconductivity greater than 1,700 Wm⁻¹K⁻¹.
 22. The graphene composite thinfilm composition as defined in claim 1 wherein said thin film has anelectrical conductivity greater than 3000 S/cm.
 23. The graphenecomposite thin film composition as defined in claim 1 wherein said thinfilm has an electrical conductivity greater than 3000 S/cm and a thermalconductivity greater than 1,000 Wm⁻¹K⁻¹.
 24. The graphene composite thinfilm composition as defined in claim 1 wherein said thin film has anelectrical conductivity greater than 1,500 S/cm, a thermal conductivitygreater than 600 Wm⁻¹K⁻¹, a physical density greater than 1.4 g/cm³, anda tensile strength greater than 10 MPa.
 25. The graphene composite thinfilm composition as defined in claim 1 wherein said thin film has anelectrical conductivity greater than 2,000 S/cm, a thermal conductivitygreater than 800 Wm⁻¹K⁻¹, a physical density greater than 1.8 g/cm3, anda tensile strength greater than 40 MPa.
 26. The graphene composite thinfilm of claim 1, having an electrical conductivity greater than 3,000S/cm, a thermal conductivity greater than 1,500 Wm⁻¹K⁻¹, a physicaldensity greater than 2.0 g/cm³, and a tensile strength greater than 40MPa.
 27. A graphene composite composition composed of nano grapheneplatelets (NGPs) bonded by a graphene oxide binder obtained from agraphene oxide gel, wherein said NGPs contain single-layer graphene ormulti-layer graphene sheets having a thickness from 0.335 nm to 100 nm,and said NGPs occupy a weight fraction of 1% to 99.9% of the totalcomposite weight; and said graphene oxide binder, having an oxygencontent of 0.1%-40% by weight based on the total graphene oxide weight,occupies a weight fraction of 0.1% to 99% of the total composite weight.28. A process for producing a graphene composite film of claim 1, saidprocess comprising: (a) preparing single-layer or multilayer grapheneplatelets from a graphitic material; (b) preparing a graphene oxide gelhaving graphene oxide molecules dispersed in a fluid medium, whereinsaid graphene oxide gel is optically transparent or translucent; (c)mixing said graphene platelets in said graphene oxide gel to form acomposite gel; and (d) forming said composite gel into said compositethin film by removing said fluid medium.
 29. The process of claim 28,wherein said graphene platelets are pristine graphene containing nooxygen.
 30. The process of claim 28, wherein said graphene platelets arepristine graphene containing no oxygen and said pristine graphene isprepared from a graphitic material without involving oxidation.
 31. Theprocess of claim 28, wherein said graphene oxide gel is prepared byimmersing a graphitic material in a powder or fibrous form in anoxidizing liquid to form an optically opaque suspension in a reactionvessel at a reaction temperature for a length of time sufficient toobtain a graphene oxide gel that is optically transparent ortranslucent, wherein said graphene oxide gel is composed of grapheneoxide molecules dispersed in an acidic medium having a pH value of nohigher than 5 and said graphene oxide molecules have an oxygen contentno less than 20% by weight.
 32. The process of claim 28, wherein saidgraphene platelets are produced from a graphitic material selected fromnatural graphite, artificial graphite, meso-phase carbon, meso-phasepitch, meso-carbon micro-bead, soft carbon, hard carbon, coke, carbonfiber, carbon nano-fiber, carbon nano-tube, or a combination thereof.33. The process of claim 28, wherein said graphene oxide gel is preparedby immersing a graphitic material in an oxidizing agent to form anoptically opaque suspension and allowing an oxidizing reaction toproceed until an optically transparent or translucent solution isformed, and wherein said graphitic material is selected from naturalgraphite, artificial graphite, meso-phase carbon, meso-phase pitch,meso-carbon micro-bead, soft carbon, hard carbon, coke, carbon fiber,carbon nano-fiber, carbon nano-tube, or a combination thereof.